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Jul 15, 2013 - Because different semiconductors exhibit different band alignments, the charge-carrier separation can be tailored by the choice of mate...
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Determination of Electronic Energy Levels in Type-II CdTe-Core/ CdSe-Shell and CdSe-Core/CdTe-Shell Nanocrystals by Cyclic Voltammetry and Optical Spectroscopy Xuedan Ma,† Alf Mews, and Tobias Kipp* Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ABSTRACT: Two different sets of type-II core/shell semiconductor nanocrystals (NCs) are prepared and studied by optical absorption, time-resolved and time-integrated photoluminescence (PL) spectroscopy, as well as cyclic voltammetry (CV). In particular, NCs with a CdTe core and a CdSe shell of variable thickness, where the holes are localized in the core and the electrons are mainly in the shell, are compared with their inverse system of a CdSe core and a CdTe shell. All measurements are correlated to model calculations based on the effective mass approximation (EMA). The comprehensive study reveals a good congruence between optical and electrochemical measurements and theoretical modeling. In particular, we find a good coincidence between the shell-thickness dependence of the band gap as measured in PL experiments, as determined from CV data, and as calculated. Interestingly, the cyclic voltammograms, which also allow for the determination of absolute electronic energy levels, are rich in features: for various shell thicknesses, several reduction and oxidation features are observed. The comparison of the energy positions and intensities of the CV signals with calculated energy levels and probability density functions from the EMA for different shell thicknesses reveals that several CV signals can be attributed to reduction or oxidation via quantized electronic ground or excited states of the type-II core/shell NCs, while others are assigned to surface states.

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INTRODUCTION Heterostructured semiconductor nanocrystals (NCs) commonly display novel optical properties that cannot be achieved with only one component.1 Whereas in conventional type-I core/shell NCs both the electron and hole wave functions are confined in the same spatial location, type-II core/shell NCs are designed so that the conduction and valence bands of the core and shell semiconductor materials are staggered at the heterojunction. Photoexcitation of charge carriers in type-II core/shell NCs might thus be followed by a charge transfer across the interface between the core and shell materials, leading to spatially separated electrons and holes. Because different semiconductors exhibit different band alignments, the charge-carrier separation can be tailored by the choice of materials for core and shell and by their dimensions. By tailoring the charge-carrier separation one can manipulate the radiative lifetime of electrons and holes confined in NCs. For example, a prolonged lifetime might allow for an effective transfer of charges from the shell material into a surrounding matrix prior to a radiative recombination. Furthermore, the type-II band alignment renders it possible to manipulate the photoluminescence (PL) wavelength of the core/shell NCs toward a range that would otherwise not be available with monocomponent nanostructures or type-I NCs. Because of these attractive optical properties, type-II core/shell NCs are potential candidates to be utilized in a number of applications such as photovoltaic technologies,2,3 lasers,4,5 and biological imaging.6 © 2013 American Chemical Society

The potential applications of type-II core/shell NCs have triggered increasing interest in the past few years,1 and CdTe/ CdSe,7−11 CdTe/CdS,12−15 and CdSe/ZnTe7,16,17 are some of the most investigated type-II material systems. These studies have provided a wealth of insight into the chemical and photophysical properties of type-II NCs, but there are still challenging issues to be addressed. For example, most of the investigations rely on optical absorption and PL measurements which yield information on the relative band-gap energies but do not allow for the determination of the absolute energy levels. However, the absolute electronic energy levels of the type-II NCs are crucial for several applications in optoelectronic devices that rely on charge-transfer processes across the surface, such as NC−polymer hybrid light-emitting diodes18,19 and solar cells.3,20 To probe absolute energy level positions with respect to a standard potential, electrochemical methods such as cyclic voltammetry (CV) have been successfully employed for CdS,21 CdSe,22−31 CdTe,27,31−33 and PbSe34 NCs. In general, such electrochemical methods are based on chargetransfer processes between electrodes and the electroactive species. However, in particular, it remains an open question as to which extent CV probes local surface states of the NC or delocalized electron or hole states, which are also probed by optical spectroscopy.31 This issue is especially important for Received: May 7, 2013 Revised: June 28, 2013 Published: July 15, 2013 16698

dx.doi.org/10.1021/jp404556b | J. Phys. Chem. C 2013, 117, 16698−16708

The Journal of Physical Chemistry C

Article

Optical Spectroscopy. UV−vis absorption and PL spectra were recorded with a Varian Cary 50 UV−vis spectrophotometer and a Varian Cary Eclipse fluorescence spectrometer, respectively. PL QYs were determined using Rhodamine 6G in ethanol as reference. PL decay curves were recorded with a home-built confocal laser microscope. Excitation laser pulses with a wavelength of 470 nm were focused by an oil objective (Zeiss, 100×, NA = 1.25). PL from ensemble NCs deposited on glass cover slides was collected by the same objective and directed to a single-photon avalanche diode. For each sample, decay curves from at least five different positions on the cover slides were recorded. Cyclic Voltammetry. The absolute band-edge energy levels of the core/shell NCs were measured by the CV method on a potentiostat/galvanostat (HEKA PG 310). To minimize the influence from moisture and air, we carried out the CV measurements in a predried five-necked electrochemical cell under argon. A three-electrode configuration was used, where a platinum disc was used as the working electrode, and a platinum wire and a silver wire served as the counter and quasireference electrodes, respectively. A 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in dry acetonitrile was used as the supporting electrolyte. Ferrocene (Fc) was added to use the Fc+/Fc couple as an internal potential standard to which all CV measurements have been referenced. Before each CV scan, ∼10 μL of concentrated NC solution was drop-casted on the platinum working electrode. For each CV scan a triangular voltage sweep between working and counter electrode with an amplitude of 2 V, that is, from 0 to 2 V, then to −2 V, and back to 0 V, was applied. For the measurements reported here, the scan rate was set to 50 mV/s; different scan rates between 20 and 200 mV/s did not lead to significant changes in the position of the oxidation and reduction peaks. The reproducibility of the results was checked by repeating the CV experiments for each sample at least five times. Between scans, the working electrode was thoroughly cleaned to remove contamination.

type-II NCs, where either the photogenerated electrons or holes are located close to the surface. Hence, in this work, we have synthesized and investigated two sets of different type-II NC systems, that is, CdTe-core/ CdSe-shell as well as CdSe-core/CdTe-shell NCs with different shell thicknesses. We have studied their optical properties by absorption and PL spectroscopy as well as their electrochemical properties by CV. For both kinds of NC systems, we correlate both kinds of measurements and compare their results to model calculations based on the effective mass approximation (EMA). We show that the changes of the PL peak energy with increasing shell thickness are in good agreement with the model calculations for both NC systems. For the CdTe-core/CdSeshell NC system we measured an increase in the PL lifetime that is in good congruence with the decrease in the calculated electron−hole overlap integral caused by the evolving localization of holes in the NC core and electrons in the NC shell. For the inverse system, that is, for CdSe NCs with CdTe shells, the quantum yield (QY) is drastically decreased, as can be explained by a large susceptibility of holes, localized in the shell, to trap states on the NC surface. For both kinds of NC systems, CV measurements reveal reduction and oxidation peaks. A band gap, as measured in CV, can be deduced and correlated to the optical band gap, as measured in PL. In particular, for the CdTe-core/CdSe-shell NC system that is structurally more homogeneous than its inverse counterpart, a good congruence between measured CV signals and calculated conduction band (CB) and valence band (VB) states is found. Here the chargecarrier separation induced by the type-II band alignment leads to effects that do not exist in simple core or type-I core/shell NCs: With increasing shell thickness, the ground state of the VB localized in the core is increasingly shielded from the NC surrounding and thus is insusceptible to oxidation during the CV. Furthermore, we observe oxidation signals that might be related to higher excited and thus less localized VB states. These signals are discussed within the theoretical model by assuming that the probability of finding the particle outside the NC is a measure of the susceptibility of the corresponding state for oxidation. For the structurally more inhomogeneous CdSecore/CdTe-shell NC system, the CV measurements also principally agree with the model calculations. Here, additionally, reduction peaks within the band gap occur that might arise from defect states.

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THEORETICAL MODELING Our theoretical modeling of the electronic level structure in CdTe-core/CdSe-shell and CdSe-shell/CdTe-core NCs is based on the EMA. Figure 1 sketches the supposed alignment of conduction and valence bands (CB, VB) exemplarily for the CdTe-core/CdSe-shell NC system. We assumed bulk band gaps of Egap(CdTe) = 1.43 eV and Egap(CdSe) = 1.74 eV, a VB offset between CdTe and CdSe of 0.57 eV, and effective masses for electrons, heavy holes, and light holes of me (CdTe) = 0.11m0, me (CdSe) = 0.13m0, mhh (CdTe) = 0.63m0, mhh (CdSe) = 0.45m0, mlh (CdTe) = 0.12m0, and mlh (CdSe) = 0.13m0, respectively.38−40 The band offset of the solution was assumed to be 3.50 eV. The core diameters of CdTe-core/ CdSe-shell and CdSe-core/CdTe-shell NCs were set to 3.43 and 2.25 nm, respectively. The average thickness of a ML of shell material was taken as 0.35 nm. The wave function ψ(r,θ,ϕ) of a particle in a sphere separates into a radial Rn,l(r) and an angular Yl,m(θ,ϕ) part, with n, l, and m being the quantum numbers and Yl,m(θ,ϕ) being the surface spherical harmonics. Using the software Wolfram Mathematica, we numerically solved the resulting radial Schrö dinger equation, assuming Ben Daniel-Duke boundary conditions.41 Figure 1 exemplarily shows the two (six) lowest electron (hole) eigenenergies together with their corresponding squared radial

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EXPERIMENTAL SECTION Nanocrystal Synthesis. The core/shell NCs used in this study were synthesized by a procedure modified from the standard SILAR method.35 In brief, CdTe and CdSe core NCs were first prepared following the method from ref 36. From the first absorption peaks at around 570 nm (see Figure 2a) and 480 nm (see Figure 3a) in the UV−vis absorption spectra, the sizes of the CdTe and CdSe core NCs were estimated to be about 3.4 and 2.3 nm, respectively.37 Subsequently, up to 5 monolayers (ML) of CdSe shell were grown onto the CdTe core NCs by alternating injections of the Cd and Se precursors into the solution containing the core NCs, whereas up to 4 ML of CdTe shell were grown onto the CdSe core NCs by alternating injections of the Cd and Te precursors. During the synthesis, trioctylphosphine oxide (TOPO) was added as surfactant to passivate and protect the NC surfaces and to prevent aggregation with direct contact upon deposition on a substrate. 16699

dx.doi.org/10.1021/jp404556b | J. Phys. Chem. C 2013, 117, 16698−16708

The Journal of Physical Chemistry C

Article

Figure 2. (a) UV−vis absorption (black) and PL (red) spectra of CdTe core NCs coated with 0−5 ML of CdSe shell. The respective spectra are vertically shifted for clarity. The PL curves have been normalized to their maxima. (b) Representative TEM image of CdTe core NCs coated with 4 ML of CdSe shell. (c) PL QYs of CdTe core NCs coated with 0−5 ML of CdSe shell.

Figure 1. Schematics of the CB and VB alignments for the case of the CdTe/CdSe core/shell NC system. The numbers give band gaps and offsets assumed for the calculations. The dashed lines represent the lowest two (six) energy states for electron (holes). Superimposed, the corresponding squared radial wave functions |R(r)|2 are shown. Panels on the right give magnified depiction of |R(r)|2 close to the NC surface for the first electron state (top) and the sixth hole state (bottom). The red shaded region corresponds to the outside of the NC.

with a 4-ML-CdSe shell shown in Figure 2b. The TEM image reveals that the NCs are monodisperse in size and uniform in shape. In addition, from the UV−vis absorption spectra it can be observed that the relative oscillator strength of the first absorption feature decreases with an increase in the CdSe shell thickness. We note that both the absorption and PL spectra are very similar to the ones very recently reported by Cai and coworkers for similar NCs.43 In particular, for absorption, the first excitonic peak position, the existence of a well-defined second excitionic peak, and its energy position coincide well with ref 43. Deviations in the distinctiveness of the second (and even higher) excitonic peaks can be explained by a slightly higher homogeneity of our samples that also manifests itself in slightly sharper first excitonic absorption peaks as well as sharper PL emission peaks. With the coating of the CdSe shell, the QY of the NCs increases from 13% for the CdTe core NCs to 20% after coating with one ML of CdSe, as shown in Figure 2c. This increase in QY is probably due to a better surface passivation of the CdTe core NCs by the growth of the CdSe shell which eliminates surface trapping states, most likely hole traps, and decreases the nonradiative pathways. When a shell of more than 1 ML CdSe is formed, the QY of the NCs decreases gradually (9% for 5 ML). This decrease can be explained by an increase in crystal strain-induced defects at the CdTe/CdSe heterojunction,44 or by a reduced electron and hole wave function overlap integral, as we will discuss in the following. Time-resolved PL measurements of the NC samples yielded multiexponential components shortly after excitation, followed by a monoexponential tail. The PL lifetimes τ of the latter continuously increased from ∼15 ns for the CdTe core NCs to above 40 ns for a 5 ML CdSe shell. Such an increase has been previously reported in ref 9. Under the assumption that these lifetime measurements only probe the single excitonic “on” state of emitting NCs with a QY approaching unity,9,45,46 for the radiative recombination rate kr ≈ 1/τ holds. Theoretically, kr is proportional to the electron−hole wave function overlap integral,42 which can be defined as |∫ R*e R*h r2 dr|2, where Re(r) and Rh(r) are the ground-state electron and hole radial wave functions, respectively. Using the effective mass model previously described,41 we calculated Re(r) and Rh(r) of the CdTe-core/CdSe-shell NCs with different shell thicknesses. With the increase in the CdSe shell thickness, the hole stays localized in the CdTe core, while the electron radial probability

wave function |R(r)|2 for a CdTe NC with a shell of 3 ML CdSe. The results of the calculations can be compared with several features of the NCs accessible by our measurements: (i) The energies of the electronic states in dependence of the shell thickness should correlate to the absorption as well as PL peaks. In particular, the calculated development of the shell-thicknessdependent energy gap between the electron and hole ground states (hatched wave functions in Figure 1) should in first approximation correspond to the behavior of the PL peak energy, even though Coulomb interactions are not taken into account in the calculations. (ii) The development of the overlap integral |∫ Re*(r)Rh(r)r2 dr|2 of the electron and hole ground states with varying shell thickness should correlate to the measured recombination rate of the PL emission.42 (iii) The absolute position of the electron- and hole-state energies in dependence of the shell thickness should also correlate to the reduction and oxidation potentials of NCs measured in CV experiments. (iv) Because CV is a surface sensitive measurement, the probability density of a certain state integrated 2 2 outside the NC, that is, ∫ ∞ d/2|R(r)| r dr (compare red shaded regions in Figure 1), should be linked to the susceptibility of oxidation/reduction and thus also be correlated to the CV measurements.

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RESULTS AND DISCUSSION Optical Spectroscopy. Figure 2a shows UV−vis absorption (black curves, greyed area) and normalized PL (red curves) spectra of the CdTe core and CdTe-core/CdSe-shell NCs. With the increase in the CdSe shell thickness, the first absorption feature continuously red shifts from 570 to 714 nm, while the corresponding PL peak red shifts from 588 to 734 nm. The relatively sharp, distinct absorption features in the UV−vis spectra and the single, narrow (